In industrial power systems, a High-speed Bus Transfer System (HSTS) scheme is applied to transfer critical loads from one source to another source during disturbances. Under normal running conditions, the loads, predominantly induction motors, are continuously connected to a source which may be called as the Main source. On failure of the Main source, the loads are transferred to a Standby source. Attempts have been made to re-connect the load to the Standby power as quickly as possible. This is to ensure that the voltage, frequency and phase angle differences across the breaker poles are within synchronizing limits, to reduce shock to the induction motors and other connected loads. This is called ‘Fast’ transfer. Though this is a desirable type of transfer, it may be a challenge to achieve this. This is because, the angle of the phase voltages induced in the motor might have already gone out of synchronization (synch) with the standby source, or a reasonable time (say 50-100 mSec) may not be available to check synchronization and breaker closing such as when droop in frequency and hence the phase angle rate of change at the load bus is too fast to allow such a time.
If a ‘Fast’ transfer opportunity is missed, motors start to slow down. The induced voltage phase angle would drift further away from the standby incoming source voltage. An attempt may be made to re-synchronize when the drift completes 360 degrees, when the two voltages are nearly in phase again. This period when the drift completes 360 degree, is referred to as one slip cycle, and is greater than the source AC power cycle. Re-synchronization is possible only when the load bus voltage magnitude and frequency are above critical levels. This method is designated as transfer at “First phase co-incidence” or “In phase” transfer and is known.
If the above condition is also not met, the motors spin down further until the voltage collapses to a very low voltage, say 20%, before attempting to reconnect to a standby source. The challenge in such a scenario is related to dealing with heavy starting current surge, mechanical shocks to connected loads, etc.
Thus, it would be desirable to have a method and system that provide efficient and fast transfer of load.